In an internal combustion engine, as one of the countermeasures for treating the exhaust gas for the purpose of decreasing noxious components produced by the combustion in the combustion chamber thereof, a method has been employed, in which secondary air is introduced at a position near the exhaust valve in order to oxidize the noxious components by supplying ample oxygen while the exhaust gas is still at higher temperature to thereby make the noxious components non-noxious. This method has been carried out either by way of so-called "air suction", in which the secondary air is conducted from the air cleaner into the exhaust port through, as the working medium of the negative pressure in the intake pipe, an air switching valve (ASV) and a vacuum transmitting valve (VTV), or by way of so-called "air injection", in which the secondary air is introduced into the exhaust lines from the compressed air from an engine-driven air pump by the control of an air switching valve also under the use of the negative pressure in the intake pipe as the control medium.
Heretofore, the introduction of secondary air was made, in general, for all the cylinders of the internal combustion engine.
In the case of an abrupt deceleration of the engine, however, the unburned gas in the exhaust system may reach an ignitable fuel-air ratio by the introduction of the secondary air, so that it is brought into a state in which the after-fire can occur to cause an unsteady operation of the engine that results in a significant false influence upon the feeling of driving of the automobile. For this reason, it has been carried out, that atmospheric air is introduced into all the intake lines of the cylinders responding to the exhaust lines fed by the secondary air, so as to bring the interiors of the exhaust lines to a state of leaner fuel-air ratio.
However, according to the above prior method, the region of occurence of the after-fire is wide and, above all, in the case of dual type exhaust manifold, though the torque here may be improved, the tendency of occurence of after-fire in the exhaust lines becomes large particularly for air suction type.
On the other hand, in the case of introducing atmospheric air into the intake lines of all the cylinders under the control of the mixture control valve (MCV) by using the negative pressure in the intake pipe as the control medium, the effect is weakened, by the diversification of the requisite amount of air to all the cylinders, that required by the constructive limitation of said valve, or it may be experienced, that inconveniences in the operation of the engine, such as deceleration surge, misfiring and breathing etc., may occur by introducing secondary air into the cylinder, whose combustion state requires no introduction of air, which in the end results in also a false influence upon the feeling of driving of the automobile.